Here's another way that builds an array of random positions in which to place a '?' and then makes the substitutions using an array slice. Downside is the spliting and rejoining of the string.

use strict;
use warnings;

my $str = q{ABC} x 10_000;
my @chars = split m{}, $str;

my @randPosns = grep { rand 10 < 1 } 0 .. length($str) - 1;
@chars[@randPosns] = (q{?}) x @randPosns;
my $newStr = join q{}, @chars;

print $newStr;
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Cheers,

JohnGG